Xeroradiography, as disclosed in U.S. Pat. No. 2,666,144, is a process wherein an object is internally examined by subjecting the object to penetrating radiation. A uniform electrostatic charge is deposited on the surface of a xerographic plate and a latent electrostatic image is created by projecting the penetrating radiation, such as X-rays or gamma rays, through the object and onto the plate surface. The latent electrostatic image may be made visible by contacting the latent electrostatic image on the plate surface with fine powdered particles (toner) electrically charged opposite to the latent electrostatic image pattern on the plate in order to develop a positive image (in order to develop a negative image, the toner is of the same polarity as the latent electrostatic image pattern). The visible image may be viewed, photographed or transferred to another surface where it may be permanently affixed or otherwise utilized. The entire processing is dry, and no dark room is necessary.
Xeroradiography in recent years has been utilized to examine the extremities, the head, and to detect breast cancer in women. In examination of breasts wherein soft tissue comprises most of the breast area, xeroradiography, or xeromammography as it is generally called, provides greater resolving power than the conventional roentgenographic film and greater image detail is achieved. A wide range of contrast is seen on the xeroradiographic plate as compared to the conventional roentgenographic films so that all the structures of the breast from the skin to the chest wall and ribs may be readily visualized. Besides providing better contrast, xeromammography detects small structures like tumor calcification and magnifies them more than conventional film, is quicker, less expensive, gives greater detail and requires less radiation than prior nonphotoconductive X-ray techniques.
The technique of powder cloud development, as disclosed in U.S. Pat. No. 2,711,481, has been utilized to develop xeroradiographic plates. This development technique is preferred in xeroradiography because discontinuities in the object being examined are readily developed. The charged surface of the plate is disposed facing a chamber area in which a cloud of powder particles are introduced. In the positive mode of development, the particles must be charged opposite to the polarity of the charge on the plate so that the particles may deposit upon the surface of the plate in an image configuration due to the action of the electrostatic forces of the latent electrostatic image of the plate acting on the charged particles in the powder cloud. In the negative imaging mode, toner of the same polarity as the latent image is attracted to the plate by means of the plate substrate voltage. Various prior art techniques for charging the powder cloud include turbulently flowing the powder particles in air through a nozzle, tube or the like to triboelectrically charge the particles or by passing the particles through a corona discharge area comprising a fine needle or fine wire and a grounded electrode as disclosed in U.S. Pat. No. 2,725,304.
U.S. Pat. No. 3,640,246 describes a powder cloud apparatus for developing latent electrostatic images wherein an ion cloud and powder cloud are introduced into the development chamber through opposite walls and meet under a baffle, extending between the opposed walls, whereby the clouds are thoroughly mixed. A development, or grid, electrode, positioned between the plate carrying the latent electrostatic image and the baffle, is included, the grid being utilized to control image contrast and quality. The grid, by appropriate biasing during the development cycle, separates particles charged to an undesired polarity and accelerates the particles of the desired polarity to the surface of the photoconductor.
The development chamber described in the aforementioned patent, although satisfactory in most respects, has certain deficiencies associated therewith. For example, utilization of the stationary close spaced grid electrode reduces image quality due to the tendency of lines developing on the image as a result of focusing the particles through the stationary grid wires. Further, the grid wires, after a relatively long development cycle, accumulates toner thereon, impairing the effectiveness of the grid by reducing toner penetration through the grid due to the narrowing of wire to wire spacing. Although periodic cleaning of the stationary grid wires would minimize the aforementioned problem, it has been found that providing a movable cleaning mechanism to periodically contact and clean the stationary grid wires usually complicates the development cycle and increases the costs associated therewith.